Method of improving the flatness of a microdisplay surface, liquid crystal on silicon (LCoS) display panel and method of manufacturing the same

ABSTRACT

A method of improving the flatness of a microdisplay surface. A reflective mirror layer and a raised layer are formed in order on a semiconductor substrate. The raised layer may comprise a buffer layer and a stop layer, and pixel electrode areas are defined therefrom and gaps are consequently formed among the pixel electrode areas. A dielectric layer is deposited on the pixel electrode areas and fills the gaps. A dielectric layer is partially removed such that the portion on the raised layer is completely removed and the portion filling the gaps are partially removed, thereby the remaining dielectric layer in the gaps has a height not lower than the top of the mirror layer. Thereafter, the raised layer is entirely or partially removed. A transparent conductive layer may be further combined onto the semiconductor substrate and a liquid crystal filling process is performed to form an LCoS display panel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention provides a method of improving the flatness of amicrodisplay surface and a liquid crystal on silicon (LCoS) displaypanel and method of manufacturing the same, and particularly an LCoSdisplay panel with a good liquid crystal arrangement to improve opticalperformance.

2. Description of the Prior Art

LCoS (liquid crystal on silicon) display technology is the key toreflective LCD projectors and rear-projection televisions. The LCoSdisplays have the advantages of tiny size, high resolution, low power,low cost, etc. The difference between an LCoS display and an TFT-LCD isthat the TFT-LCD uses a glass substrate or a quartz substrate as theback plane and is provided with a light source from the back side, whilethe LCoS display uses a silicon substrate as the back plane and isprovided with a light source from the front side. As a result, themanufacturing of LCoS display panels can be integrated into standardsemiconductor processes, and therefore has high stability andresolution.

However, in conventional LCoS display panels, the liquid crystalmolecules in the liquid crystal layer tends to be toppled due to therecess between the mirror layer units. Referring to FIG. 1, asemiconductor substrate 12 comprises transistor devices (such as CMOS),metal interconnects, and metal plugs (not shown). A plurality ofreflective mirror layer units 14 are formed on the surface of thesemiconductor substrate 12. A dielectric layer 16 is filled in the gapbetween the mirror layer units 14. A transparent electrode 22 and atransparent substrate 24 are combined with the semiconductor substrate12. Liquid crystal layer 26 is filled in the cell gap between thetransparent electrode 22 and the semiconductor substrate 12. It is notedthat a surface on which the liquid crystal layer 26 stands is comprisedof a reflective mirror layer 14 and the dielectric layer 16. The surfacehas a recess at the position of the dielectric layer 16 due to theprocess. The liquid crystal molecules between the two pixel electrodeareas are toppled due to the recess, resulting different arrangementangles for the liquid crystal molecules. Thus, the optical performancefor the location between two pixels is affected.

Refer to FIG. 2 illustrating a conventional method of manufacturing LCoSdisplay panels. In the conventional method, after reflective mirrorlayer units are defined, a dielectric layer 16 a is deposited to fillthe gaps between the mirror layer units. Due to processes, a recess 18occurs on the surface of the dielectric layer corresponding to the gapsbetween the mirror layer units 14. Thereafter, referring to FIG. 3, aCMP process is generally used to remove part of the dielectric layer,and an etching process is continued to partially remove the resultingdielectric layer 16 a, for not damaging the mirror layer units 14(generally, aluminum). To ensure entire removal of the dielectric layeron the mirror layer units 14, an over etching is performed to leave thedielectric layer 16 only in the gaps; and the recess 20 is accordinglyformed. The recess of the dielectric layer in the gaps is caused byprocess uniformity, similar to the inter layer dielectric (ILD)planarization in a general IC process. The recess 20 may have a depth,R, of more than 500 Å, and the arrangement of liquid crystal isaffected.

U.S. Pat. No. 6,569,699 discloses a method of fabricating aLCD-on-silicon pixel device, in which, a pixel electrode is made byforming a first opaque conductive layer (comprising for example W, Ti,TiN, Cr, Ag, Co, or CoN) over the ILD, completely filing the via,planarizing the surface of the first opaque conductive layer having therecess caused by the plug, forming a second opaque conductive layer(comprising for example aluminum (Al), an aluminum copper alloy (AlCu),or aluminum silicon copper alloy (AlSiCu)) over the first opaqueconductive layer, defining the pixel electrodes, and forming an opticalinterface layer as a reflective layer (for example, ON (i.e. an oxidelayer over a nitride layer), ONON, ONONON, etc., up to five ON pairs) onthe pixel electrodes. This patent intends to improve the lightreflectivity of the pixel electrodes, while gaps exist between the pixelelectrodes.

U.S. Pat. No. 6,750,931 discloses a reflective liquid crystal device, inwhich, four dielectric layers having different refractive indices areformed in order directly on the pixel electrode surface to improve lightreflectivity. The problem of a recess on the surface on which liquidcrystal molecules stand is not mentioned.

Udayan Ganguly and J Peter Krusius in “Fabrication of UltraplanarAluminum Mirror Array by Novel Encapsulation CMP for Microoptics andMEMS Applications,” Journal of The Electrochemical Society, 151 (11)H232-H238 (2004), disclose a method of manufacturing pixel electrodes ofa micro-display by encapsulation CMP. Unlike a general technique toforming a metal layer on the planar ILD to make pixel electrodes, inthis method, a plurality of trenches are formed by etching the ILD, ametal layer is deposited conformally in the trenches, and a cap layer isdeposited conformally over the metal layer, similar to encapsulate themetal layer in a plurality of trenches. Next, a part of the cap layeraround the die is removed by reactive ion etching (RIE), and then thehigher portion of the cap layer on the trench wall is removed by CMP.Finally, the part of the metal layer located on the trench wall higherthan the bottom of the trench is removed by CMP, to achieve a planarsurface. The surface comprises a cap layer and a small portion of themetal layer revealed from the cap layer.

Therefore, a LCoS display panel and the manufacturing method to solvethe problem of the recess of the surface on which the liquid crystalmolecules stand in order to improve optical performance of the displaydevice are still needed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of improvingthe flatness of a microdisplay surface, an LCoS display panel and amethod of manufacturing the LCoS display panel, such that the liquidcrystal molecules therein are well arranged to have an improved opticalperformance.

The method of improving the flatness of a microdisplay surface accordingto the present invention comprises the steps as follows. First, asemiconductor substrate is provided. A reflective mirror layer is formedon the semiconductor substrate. A raised layer is formed on the mirrorlayer. Next, a plurality of pixel electrode areas is defined from theraised layer and the mirror layer; thereby a gap is formed between eachtwo adjacent pixel electrode areas. A dielectric layer is deposited onthe pixel electrode areas, completely fills the gaps. Thereafter, thedielectric layer is partially removed thereby the portion of thedielectric layer on the surface of the raised layer is completelyremoved and the portion of the dielectric layer filling the gaps ispartially removed such that the remaining dielectric layer in the gapshas a height not lower than the top of the mirror layer. The raisedlayer is partially or entirely removed.

In another aspect, the method of manufacturing LCoS display panelsaccording to the present invention comprises the steps as follows.First, a semiconductor substrate is provided. A reflective mirror layeris formed on the semiconductor substrate. A raised layer is formed onthe mirror layer. Next, a plurality of pixel electrode areas is definedfrom the raised layer and the mirror layer; thereby a gap is formedbetween each two adjacent pixel electrode areas. A dielectric layer isdeposited on the pixel electrode areas, completely fills the gaps.Thereafter, the dielectric layer is partially removed thereby theportion of the dielectric layer on the surface of the raised layer iscompletely removed and the portion of the dielectric layer filling thegaps is partially removed such that the remaining dielectric layer inthe gaps has a height not lower than the top of the mirror layer. Theraised layer is removed. A transparent conductive layer is combined ontothe semiconductor substrate with the pixel electrode areas therebetween.Finally, a liquid crystal filling process is performed to fill theliquid crystal in a cell gap between the semiconductor substrate and thetransparent conductive layer.

In still another aspect, the LCoS display panels according to thepresent invention comprises a semiconductor substrate; a plurality ofreflective mirror layer units on the semiconductor substrate to serve aspixel electrodes and for light reflection, wherein a dielectric layer isfilled between each two adjacent reflective mirror layer units with aheight not lower than the top of the mirror layer units; a transparentconductive layer combined onto the semiconductor substrate with themirror layer units therebetween; and a liquid crystal layer disposedbetween the semiconductor substrate and the transparent conductivelayer.

In the method of improving the flatness of a microdisplay surface, theLCoS display panels and the method of manufacturing the same accordingto the present invention, since the height of the dielectric layerfilling the gaps among the pixel electrodes is equal to or slightlyhigher than the height of the pixel electrodes, the surface such formedhas not recesses. Accordingly, the arrangement of the liquid crystalmolecules is not affected and has a good light performance.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematically cross-sectional diagram showing a conventionalLCoS display panel;

FIG. 2 is a schematically cross-sectional diagram showing a structureafter a dielectric layer is filled between the mirror layer units in aconventional method of manufacturing LCoS display panels;

FIG. 3 is a schematically cross-sectional diagram showing a structureafter the dielectric layer is removed from the mirror layer units in aconventional method of manufacturing LCoS display panels;

FIGS. 4-8 are schematically cross-sectional diagrams showing the methodof improving the flatness of a microdisplay surface and the method ofmanufacturing LCoS display panels according to the present invention;

FIG. 9 is a schematically cross-sectional diagram showing anotherembodiment according to the method of the present invention;

FIG. 10 is a schematically cross-sectional diagram showing an embodimentof the LCoS display panel according to the present invention; and

FIG. 11 is a schematically cross-sectional diagram showing anotherembodiment of the LCoS display panel according to the present invention.

DETAILED DESCRIPTION

Please refer to FIGS. 4-8 which are schematically cross-sectionaldiagrams showing the method of improving the flatness of a microdisplaysurface according to the present invention. As shown in FIG. 4, first asemiconductor substrate 32 is provided. For example, a silicon substratemay be used as the semiconductor substrate. The semiconductor substratecomprises a controlling circuit (not shown). The controlling circuitcomprises metal interconnects and a plurality of transistors, such asCMOS transistors, arranged in an array for driving pixel electrodes. Thecontrolling circuit connects the pixel electrodes on the surface of thesemiconductor substrate through via plugs.

Next, a reflective mirror layer 33 is formed on the semiconductorsubstrate 32. A raised layer 41 is formed on the mirror layer 33. Theraised layer may have a structure of a single layer formed of onematerial or a composite layer including a single layer formed of aplurality of materials or a multilayer formed of a plurality ofmaterials. Due to the existence of the raised layer, the dielectriclayer remained after subsequent partial removal of the dielectric layerhas an increased height, such that a recess will not occur. Accordingly,the thickness of the raised layer is not particularly limited as long asthe purpose to raise the thickness of the dielectric layer can beachieved. For the convenience to process, the thickness of thedielectric layer may be for example 100 to 1000 Å. Particularly, abuffer layer 35 may be formed on the mirror layer 33 and the stop layer37 may be formed on the buffer layer 35 to be as a raised layer. Thestop layer functions as an indication for stop during the subsequentremoval of the dielectric layer. A material functioning the stopindication for the removal may be selected as the stop layer. Forexample, for a CMP process on the dielectric layer, titanium nitride canbe used as a stop layer, which may be formed by reactive sputtering orchemical vapor deposition. The buffer layer offers a buffer functionbetween the stop layer and the mirror layer during the subsequentprocess of removing the stop layer, for preventing the smooth surface ofthe mirror layer from being damaged. The titanium nitride layer has agood etching selectivity over the silicon oxide layer. When the stoplayer comprises a titanium nitride layer, the buffer layer may comprisesilicon oxide, which may be formed by a vapor deposition process. Othermaterial capable of serving the same function may be used.

A main function of the mirror layer is to provide reflectivity forincident light and to be a pixel electrode electrically connected withthe controlling circuit through the corresponding metal plug. Thematerial having electric conductivity and capable of forming a smoothsurface for reflection of light may be used, for example, metal, such asaluminum. The metal layer may be formed by for example sputtering, etc.

Next, please refer to FIG. 5. A plurality of pixel units are definedfrom the stop layer 37, the buffer layer 35, and the mirror layer 33,and gaps 39 between each two pixel units are consequently formed. Atthis time, the pixel unit comprises the mirror layer unit 34, the bufferlayer unit 36, and the stop layer unit 38. The buffer layer unit 36 andthe stop layer unit 38 may be deemed as the raised layer unit 40. Thedefinition of the pixel units may be accomplished by for examplemicrolithography and the etching process. Thereafter, as shown in FIG.6, a dielectric layer 42 is formed on the semiconductor substrate 32 tocover the mirror layer unit 34, the buffer layer unit 36, and the stoplayer unit 38, and to completely fill the gaps 39. The dielectric layermay be formed by for example chemical vapor deposition. The dielectriclayer such formed may have recesses 43 on the surface at the locationcorresponding to the gaps 39.

Then, referring to FIG. 7, the dielectric layer 42 is partially removedthereby the portion of the dielectric layer on the surface of the stoplayer unit 38 is completely removed and the portion of the dielectriclayer filling the gaps 39 is partially removed, such that the height ofthe dielectric layer 42 a in the gaps 39 is not lower than the top ofthe mirror layer unit 34. The partial removal may be accomplished by forexample CMP, dry etching, wet etching, or a combination thereof. Forexample, a CMP is performed to fast remove the dielectric layer on thestop layer unit 38, or a back etching process is performed, to removethe residue of the dielectric layer on the stop layer unit 38. Thus,although recesses 44 may be formed at the location of the gaps 39, therecesses 44 may be eliminated after the stop layer unit 38 or both ofthe stop layer unit 38 and the buffer layer unit 36 are removedsubsequently, as shown in FIG. 8. Therefore, in the partial removal ofthe dielectric layer 42, the dielectric layer 42 a remained in the gaps39 must have a height not lower than the top of the mirror layer unit34, such that, recesses will not occur at the location of the gapsbetween two pixel electrodes after the stop layer and the buffer layerare removed. The removal of the stop layer unit 38 may be accomplishedby, for example, a wet etching process.

There are two situations for the partial removal of the dielectriclayer. One is that the height of the dielectric layer 42 a is about thetop of the mirror layer, and after the stop layer and the buffer layerare removed, the top of the dielectric layer 42 a is as high as the topof the mirror layer to form a planar surface. Thus, the arrangement ofthe liquid crystal molecules is not affected. Another is that, as shownin FIG. 9, the height of the dielectric layer 42 b is higher than thetop of the mirror layer unit 34, and after the stop layer and the bufferlayer are removed, the top of the dielectric layer 42 b is higher thanthe top of the mirror layer to slightly bulge. Such bulge is acceptablein the subsequent processes and will not affect the arrangement of theliquid crystal.

Finally, referring to FIG. 10, a transparent conductive layer iscombined onto the semiconductor substrate 32. For example, a transparentconductive layer 46 comprising indium tin oxide (ITO) and a transparentsubstrate 48 are together combined onto the semiconductor substrate 32,and a cell gap is formed between the transparent conductive layer 46 andthe semiconductor substrate 32. Subsequently, a liquid crystal fillingprocess is performed to fill the liquid crystal in the cell gaps to fora liquid crystal layer 50, achieving an LCoS display panel.

Furthermore, please refer to FIG. 11. In the method of manufacturingLCoS display panels according to the present invention, a passivationlayer 52 may be further formed after the partial removal of thedielectric layer and removal of the stop layer and the buffer layer.Since the mirror layer generally comprises metal, such as aluminum, itcan be protected from contacting the process environment or the liquidcrystal material once being covered by a passivation layer. Thepassivation layer may comprise inert material layer, for example, oxidelayer, oxide-nitride layer (ON layer), oxide-nitride-oxide-nitride layer(ONON layer), or more ON layers. When the passivation layer comprises aproper multi-ON layer, the total reflection of the mirror layer also canbe improved.

In the method of improving the flatness of a microdisplay surface andthe method of manufacturing LCoS display panels according to the presentinvention, after partial removal of the dielectric layer, both the stoplayer and the buffer layer, or only the stop layer may be removed. Incase that only the stop layer is removed, the buffer layer remained maybe damaged during the removal of the stop layer. While, when apassivation layer is further to be formed, this buffer layer may be actas a part of the passivation layer. For example, when the buffer layeris a silicon oxide layer, in the subsequent ONON passivation layerforming process, the buffer layer may serve as a part of the oxide layerto form the oxide layer of the ONON layer, that is, the oxide layer ofONON layer is combined with the buffer layer.

Furthermore, still referring to FIG. 11, an alignment film 54 may befurther formed on the passivation layer 52 to align the arrangement ofthe liquid crystal molecules.

All combinations and sub-combinations of the above-described featuresalso belong to the present invention. Those skilled in the art willreadily observe that numerous modifications and alterations of thedevice and method may be made while retaining the teachings of theinvention. Accordingly, the above disclosure should be construed aslimited only by the metes and bounds of the appended claims.

1. A liquid crystal on silicon (LCoS) display panel, comprising: asemiconductor substrate; a plurality of reflective mirror layer units onthe semiconductor substrate to serve as pixel electrodes and for lightreflection, wherein a dielectric layer is filled between each twoadjacent reflective mirror layer units higher than the top of the mirrorlayer units; a transparent conductive layer combined onto thesemiconductor substrate with the mirror layer units therebetween; and aliquid crystal layer disposed between the semiconductor substrate andthe transparent conductive layer.
 2. The display panel of claim 1,wherein the mirror layer comprises metal.
 3. The display panel of claim2, wherein the metal comprises aluminum.
 4. The display panel of claim1, further comprising a passivation layer on the mirror layer.
 5. Thedisplay panel of claim 4, wherein the passivation layer comprises aplurality of layers of silicon oxide-silicon nitride layer.
 6. Thedisplay panel of claim 4, further comprises an alignment film on thepassivation layer.